Magnifier



May 12, 1959 J. G. BAKER MAGNIFIER Filed March 12, 1958 INVENTOR.wMQsQBA/(ER SEARCH RDOM United States Patent 2,885,928 MAGNIFIER JamesG. Baker, Winchester, Mass, assignor to the United States of America asrepresented by the Secretary of the Air Force Application March 12,1958, Serial No. 721,075 2 Claims. (Cl. 88-57) The present inventionrelates generally to magnifiers of wide covering power and gooddefinition and, as illustrated herein, relates more particularly to theuse of such magnifiers in stereoscopes.

Various types of magnifiers are found in the art. The simplestmagnifiers consist of one or two elements, corrected for a smallexternal pupil and, in some cases, for astigmatism and distortion.Magnifiers usually are related to eyepieces used in visual opticalinstruments. These eyepieces usually have an external, accessible pupillocation where the eye may be placed for the purpose of seeing a largereal field. In other words, the eye is located at the exit pupil of theoptical instrument regardless of its complexity. In the case of theeyepiece, only the rays passing through this well defined exit pupilneed be corrected. With such a small aperture, the system need only becorrected for astigmatism, field curvature and distortion, and forlateral color. Spherical aberration, coma and longitudinal chromaticaberrations are normally present but the eye selects so small a portionof the possible rays passing through the eyepiece that these aberrationsordinarily are not significant and hence need not be corrected for suchaberrations.

In the case of a magnifier, the object plane is illuminated by lightfrom all directions and there is no well defined pupil or stop, exceptthe one defined broadly by the magnifier itself. There is no precedingoptical instrument for collecting and focusing rays along desirablepaths. In such magnifiers, the eye may still be placed at a positionalong the optical axis external to the magnifier which would besubstantially identical with the stop used if the magnifier wereemployed as an eyepiece.

If the usual type eyepiece is used as a magnifier and is viewed with theeye in a position displaced either longitudinally or transverselythereof, the observer will see considerable amounts of distortion,astigmatism and color aberration. This situation will, be particularlyacute when the eye of the observer is oif center at the edge of thebundle of rays transmitted by the eyepiece. Thus, as a rule, eyepieceswhich are sufficiently corrected for their own purposes usually prove tobe inadequate when used as magnifiers.

However, in spite of the shortcomings of simple optical systems whichhave been designed as eyepieces, they are manufactured and distributedfor use as magnifiers also. The observer simply accepts the difficultiesand attempts to look through the part of the eyepiece which gives himthe best results. In the case of stereoscopes, the situation is greatlyaggravated. Here two eyes are involved and the observers eyes may belooking through different portions at either side of the magnifier sothat the aberrations will not only produce distortions but will blur theimage and distort the stereoscopic effect as well.

In the average small stereoscope now on the market, only simplepiano-convex elements are used. Such a lens form has some merit in thatthe amount of astigmatism is small and there is an accessible externallocation for the eye but it cannot be corrected for distortion nor, withsimple elements, for either longitudinal or lateral color. Accordingly,users of standard stereoscopes now in common use must tolerate pooroptical performance in the outer portion of the field of viewtransmitted by the single lens magnifiers.

One object of the present invention is to provide a magnifier for use ina stereoscope by the use of which the present difficulties will beovercome. To this end, the magnifier is designed as an optical systemcorrected for an accessible external pupil that would be free ofspherical aberration, coma, astigmatism, curvatures of field,distortion, and longitudinal and lateral color. This involves alsodesigning a system that would be uniformly good over a large pupil. Insuch a system, all of the emergent rays are adequately corrected and itis unimportant between relatively wide limits where the observers eyesare located either transversely or longitudinally above the magnifiers.

Another object of the invention is to improve generally upon theconstruction and operation of stereoscopic magnifiers. 3

With the above and other objects and features in view the invention willnow be described with particular reference to the accompanying drawingin which is shown a lens system constructed according to the presentinvention.

The present invention is designed particularly for use in stereoscopicmagnifiers whereby fine detail in stereoscopic maps or the like may bemore easily and conveniently studied. The rays entering the observerseyes are so well corrected that he has no feeling that his vision isimpaired at all. Accordingly, there is no eyestrain such as is usuallyassociated with stereoscopic viewing and the observers eyes are just asmuch at home in the new image space as they would be for viewing undernormal circumstances without optical aid.

The magnifiers concerned here are capable of giving overlapping fieldsof view measuring 2% inches laterally and 2 inches longitudinally. Inother words, the observer is enabled to obtain stereoscopic viewing at4X over a 2 by 2 /2 inch photograph. Over this entire entire field,there is no obvious distortion or blurring of the image. The observermay hold his head well above the magnifiers without facing the necessityof gluing his eyes to the stereoscope as would normally be the case. Theobserver may also focus the magnifiers so that the apparent image liescoincident with the top of his desk. This enables the observer to lookthrough the magnifier or to make notes without need to refocus or changethe convergence of his eyes.

The optical system of the present 4 magnifier resembles a photographicobjective comprising two components, the first adjacent to the eye iscomprised of three cemented elements and the second is comprised of acemented doublet. Normally, the outermost elements in such a system,namely, elements I and V, are negative and positive, respectively, withpositive and negative elements cemented to them. I The present system isdesigned to be uniformly good over a pupil as large as 1.2 inches indiameter.

Although a 2 magnifier might be scaled down and thereby converted to 4Xby a simple change of focal length, to do so would mean a scaling downof the pupil size also. Thus where 2 X magnifier might have a 1.30 inchdiameter pupil, the 4x version would have only a 0.65 inch diameterpupil. Although this would be reasonably good, the observer finds itmuch easier when the pupil diameter is something over 1.0 inch. The 4magnifiers constructed according to the present invention satisfy thisrequirement by providing a 1.20 inch diameter pupil. This means that theoptical system operates over a field of view of about 45 for a pupildiameter of 1.20 inches. This means that the system is operating atapproximately f/2 with an external pupil.

The performance noted above would not be unusual if a central pupil,that is, a pupil located symmetrically inside of the optical system,were involved. In the case of magnifiers, however, the pupil is externaland, hence, the optical system must be corrected for rays that do notnormally get through the system. It is also true that the best correctedrays that normally get through in symmetrical systems are not used atall. At least, the observer would have to move his head far to one sideand look obliquely through the optical system if he were to make use ofthe rays normally best corrected in a photographic system.

As stated above, the 4X magnifier consists of two components, the oneadjacent to the eye is comprised of three cemented elements and theother component being a cemented doublet. This construction may appearsimpler than the one for a 2X magnifier but it proved necessary to makeuse of two aspheric surfaces one on each component to insure sharpimagery over the desired flat field of view. Otherwise, these rays wouldnot be properly corrected and would prevent a very displeasing effect tothe observer.

Calculations showed that the fourth surface of the system R had to begiven a very strong up-edge, departing markedly from a sphericalsurface. The last surface R of the system, is provided with a fairlyweak turneddown edge. There is a strong tendency for the upper rim raysto be undercorrected. This under-correction is caused by the strongcemented surface of the rear doublet but generally cannot be reducedwithout an increase in other more objectionable aberrations. The use ofa compensating aspheric up-edge on the fourth surface overcomes thisunder-correction in the intermediate field, whereas in the outer fieldwhere the correction afforded by the aspheric up-edge becomes toostrong, the turneddown edge of higher order on the last surface becomesnecessary also, if adequate performance over the whole field is to beobtained.

The present optical system illustrated in the figure of the drawing hasonly four air surfaces and accordingly is capable of yielding a clearimage of high contrast.

The following table illustrates by way of example, an embodiment of theobjectives of the present invention which shows an astimaticallyflattened image field of view, excellent correction for coma, sphericaland both lateral and longitudinal chromatic aberration together with alarge external pupil which permits convenient and comfortable use of themagnifier with a minimum of distortion.

In accordance with the drawing, the radii of curvature of the lenssurfaces are denoted F1 R R R R R and R The axial thickness of thelenses is indicated by t t t t and t and the axial distance between thecomponents is indicated by S The lenses are denoted by I, II, III, IVand V. The glasses used are indicated by the index and the Abbe numberThe mean refraction indices for light at the D line are denoted n n n m,and n respectively.

A focal length of 2% inches has been assumed for the objective shown inthe drawing.

The cross-section curve is defined in rectangular coordinates by:

with and n in inches, E and 1 being the X and Y coordinates respectivclyof the curves surface with respect to its pole.

The order of the elements, for the doublet nearest the eye has beenreversed so that element I is formed of crown glass and element II isformed of flint glass. This was desirable since a magnifier undergoesfrequent handling and cleaning. Thus it is important that elementnearest the eye be made of as hard 3. glass as possible to reducescratching or marking of the element I to a minimum.

It is important that the optical glasses used should be water white. Theeye is sensitive to minor coloration, even though percentage wise, thereis little loss of light in the blue and violet portion of the spectrum.Even slight coloration would make a piece of white paper appear darkish.On the other hand, it is important for purposes of obtaining shallowcurvatures and reduced optical aberrations that glasses having ratherhigh indices of refraction should be used to obtain a large fullycorrected field of view. The present system illustrated in the drawingis believed to be the best compromise possible between these opposingrequirements. The high index elements used for the positive elementshave been carefully selected from among many types to be especiallyclear and free from coloration. The other elements offer no problem inthis respect.

Having thus described my invention what I claim as new and desire tosecure by Letters Patent of the United States is:

l. A lens system corrected for spherical and chromatic aberrations,coma, astigmatism and curvature of field comprising two air spacedcomponents in optical alinement with each other, said lens system beingconstructed according to the numerical data given in the table herebelowin which R to R designate the radii of the refractive surfaces of thelenses, starting at the front of the objective, t to t designate axialthicknesses of the respective lenses, S designates the axial spacebetween the lenses, n represents the index of refraction of the lensmaterial for the D line of the spectrum and 1 represents the dispersionof said lens material,

U=214 in.

4 power.)

Radli Thickness n v tit-1.617

. Aspherlc.

The cross-section curve is defined in rectangular coordinates by:

with 5 and 1 in inches, 5 and n being the X and Y coordinatesrespectively of the curves surface with respect to its pole.

2. A lens system having an external pupil and operating at a relativeaperture of at least f/ 2 and a field angle of at least 45 which iscorrected for spherical and chromatic aberrations, coma, astigmatism andcurvature of field comprising two air spaced lens components in opticalalinement with each other, said objective being constructed according tothe numerical data given in the table herebelow in which R to Rdesignate the radii of the refractive surfaces of the lenses, startingat the front of the objective, t, to t designate axial thicknesses ofthe respective lenses, 8; designates the axial space between the lenses,n represents .he index of refraction of the lens material for the D lineof the spectrum and 1' represents the dispersion of said lens material,

[f=2% in. 4 power.]

Radil Thickness nn v R 1.418 I t1=.14o m=1.517 64.5

R2- .878 II tg=. 568 flg=1. 642 58. 1

Ra=2.153 II'I t;=.1s5 m=1.617 36.6

S1=.337 Rl- 4.782 t|=. 205 m=1. 617 36. 6

R5- 1.345 V tt=-.790 m-l. 678 55.5

The cross-section curve is defined in rectangular coordinates by:

with 5 and 1 in inches, g and 1 being the X and Y coordinatesrespectively of the curves surface with respect to its pole.

References Cited in the file of this patent UNITED STATES PATENTS

